Piezoelectric materials such as PZT (lead zirconate titanate) have been used to form piezoelectric devices for use in actuators and sensors. Traditionally, bulk piezoelectric materials have been used. In recent years, thin film piezoelectric materials (piezoelectric thin films) have been studied and used to meet the need for a reduction in size and thickness. Particularly when a piezoelectric device is used as an actuator for an inkjet head, it is important to make the piezoelectric device smaller for each nozzle, because nozzles must be arranged at high density for high-definition image printing.
Methods for depositing piezoelectric thin films include sputtering and sol-gel techniques. Piezoelectric thin films are deposited on a substrate layer and therefore strongly influenced by the material, orientation, and other properties of the substrate layer. In order to improve the properties of piezoelectric thin films, therefore, various methods are used, such as control of the orientation of a lower electrode used as the substrate and placement of an orientation control layer between a lower electrode and a piezoelectric thin film.
A precious metal such as Pt, Au (gold), or Ir (iridium) is often used to form the lower electrode. In addition, a bonding layer including Ti or TiOx (titanium oxide) is often provided between the lower electrode and the substrate (such as an oxidized silicon substrate) in order to increase the bonding between them. For example, Patent Literature 1 discloses a piezoelectric device having a silicon oxide film, a titanium oxide film (bonding layer), a lower electrode including Pt, a PZT film (piezoelectric thin film), and an upper electrode, which are stacked in this order on a silicon substrate.
According to Patent Literature 1, Ti is deposited in the form of islands on the lower electrode by sputtering, and the PZT film is deposited on the Ti islands. It is suggested that when Ti is deposited in the form of islands on the lower electrode, the Ti islands can serve as crystal nuclei to control the orientation of PZT during the deposition of the PZT film on the lower electrode, so that the resulting PZT can have improved crystallinity (orientation) and thus improved piezoelectric properties.
For example, Patent Literature 2 also discloses such a method including depositing seed crystals (for serving as crystal nuclei) in the form of islands on a lower electrode and then depositing a piezoelectric thin film thereon. According to Patent Literature 2, after a bonding layer including Ti and a lower electrode including Pt are formed in this order on a substrate, the substrate is annealed in an oxygen atmosphere so that Ti is diffused into Pt and TiOx seeds are distributed in the form of islands on the lower electrode.
Actually, in this regard, even without annealing in the oxygen atmosphere, seed crystals (Ti) can precipitate on the lower electrode depending on the settings of the deposition conditions such as the substrate temperature during Pt deposition, because a sufficient amount of oxygen exists for the amount of the atoms on the thin film surface even when the pressure is around the back pressure of the vacuum system (10−3 Pa to 10−5 Pa).
Unfortunately, it is difficult to control the necessary amount of Ti by sputtering in the method of forming seed crystals (Ti) on the lower electrode by sputtering as disclosed in Patent Literature 1.
In this regard, Patent Literature 2 discloses that the ratio of seed crystals (TiOx) to Pt on the lower electrode surface is about 1.5% by atom. It is difficult to control the amount of Ti to a level of around 1.5% by atom by sputtering although the crystallinity of a perovskite crystal, such as PZT, deposited on the lower electrode significantly depends on the ratio of the seed crystals. Even when one atom thick layer of seed crystals are deposited, the deposition time is about few seconds although it depends on the supplied power and other conditions. Therefore, the deposition time would be shorter when the deposition on the surface is performed at a ratio of 1.5% by atom. Using sputtering, it is very difficult to form a uniform film on a substrate in such a short deposition time. If the resulting film is not uniform, the productivity will significantly decrease.
Alternatively, Ti can be precipitated from the bonding layer onto the lower electrode. In this method, appropriate control of the amount of Ti precipitation can be more easily achieved by the control of the Pt deposition temperature than in the method of forming seed crystals by sputtering. Therefore, this method is advantageous in that it can easily deposit Ti uniformly on the substrate.